The present invention relates to an ink image-receiving sheet which is excellent in writing property, and on which an image that is excellent in gradation reproducibility and dot reproducibility including variable performance and color clearness can be formed by using a thermal transfer printer, and a method of forming an image wherein the ink image-receiving sheet is used.
With respect to a method of forming a high-definition image through a thermal transfer printer, a method of forming an image comprising: using an image-receiving body having a porous layer, and melt-transferring ink from thermal transfer sheets of yellow, magenta, and cyan, (if necessary, black or the like) thereupon, wherein a heat-meltable ink in a molten state is infiltrated into pores in the porous layer was previously reported in Institute of Television Technical Report, ITE Technical Report, Vol.17, No.27 (May, 1993), p.19-24. Furthermore, as a method of forming a porous layer, the following are known: Wet-solidification method or wet-coagulation method (hereinafter referred to as xe2x80x9cwet-solidification methodxe2x80x9d), which is a method of forming a porous layer wherein a sheet on which a solution of a binder resin in dimethylformamide is applied is dipped into water, thereby replacing the dimethylformamide with water, see JP, B, 49-25430, and JP, A, 5-155163; Mechanically agitating foaming method, which is a method of forming a porous layer wherein a binder resin is mechanically agitated and foamed, and thereafter, the resultant is applied on a sheet, see JP, A, 7-32753, and JP, A, 7-309074; Pigment addition method, which is a method of forming a porous layer wherein a porous pigment is added into a binder resin so as to utilize the oil-absorbing property of the pigment, see JP, A, 3-98333, and JP, B2, 2535371; Solvent-dissolving method or dry method, which is a method of forming a porous layer wherein a binder resin is dissolved into a mixture of a good solvent having a low boiling point and a poor solvent having a high boiling point, and the resultant is dried so that the voids which remain after the poor solvent which is slow in drying is removed by drying form pores, see JP, A, 4-82790, and JP, A, 6-166283; Foaming agent method, which is a method of forming a porous layer wherein a foaming agent which generates a gas upon heating is added into a binder resin, see JP, A, 2-3396; Method of dissolving and removing soluble particles, which is a method of forming a porous layer wherein soluble particles are added into a binder resin and the resultant is formed into a film, and the film is then washed to remove the soluble particles, JP, A, 6-171250; and the like. However, even according to any of those methods, it is difficult to control the state of pores formed in an ink-receiving layer, which is considered to be most important in a thermal transfer of infiltration type, and as a result, there is a problem that the number of pores in the ink-receiving layer is decreased so that an infiltration of ink cannot be sufficiently attained, or a problem that the pore size becomes too large, which exerts a bad influence upon a transfer of fine dots.
Among the above-mentioned methods of forming a porous layer, the wet-solidification method is a method by which a large number of pores can be relatively easily formed in an ink-receiving layer, and thus various technologies have been investigated. Among them, technologies of adding a filler component to a binder component in order to improve the strength of the ink-receiving layer was reported in JP, B, 49-25430 (which discloses wet-solidification using a styrene resin, a plasticizer and a filler), and JP, B, 5-18332 (which discloses wet-solidification using a polyester resin, a plasticizer and a filler). However, even in ink-receiving porous layers formed according to these technologies, pores therein have a too large pore size, and the pores are coarse. Therefore, on thermal transfer printing, an infiltration of ink cannot be sufficiently attained, and thus, it is difficult to form dots in a uniform shape. In particular, when a full-color printing is carried out, a second color ink or subsequent color ink(s) which are to be overlapped with a first color ink or with each other cause an inferior transfer, and thus, it has been difficult to form a high-definition image.
The present inventors have evaluated various types of image-receiving sheets based on the prior art mentioned above. With respect to the mechanically agitating foaming method and wet-solidification method, each of which is considered to largely leave room for improvement from the view point of forming a high-quality image as compared with conventional image-receiving sheets, the characteristics of each of these methods are put in order as follows: Namely, in the case of an image-receiving sheet having an ink-receiving layer comprising a micro-porous layer formed according to the mechanically agitating foaming method, an aqueous coating liquid can be used, and the image-receiving sheet can be formed by merely drying after application of the aqueous coating liquid, and thus this type of image-receiving sheet has the advantage that a paper support can be used, and a support for the image-receiving sheet has a wide range of choice, while communicating pores have a large variation in pore size and a lot of the pores have a large pore size, and thus this type of image-receiving sheet has, for example, the disadvantage that it is difficult to form a sharp dot, small dots in a highlight region cause an outstanding rough appearance, it is difficult to provide a high maximum density, and the gloss of the obtained image is inferior. On the other hand, in the case of an image-receiving sheet having an ink-receiving layer comprising a micro-porous layer formed according to the wet-solidification method, a support of pure paper cannot be used due to the process, and manufacturing costs are high, but from the view point of forming a high-quality image, communicating pores have a small variation in pore size and a little of the pores have a large pore size as compared with the mechanically agitating foaming method, and thus the performance is largely improved. However, in order to stand comparison with a sublimation transfer method or to be superior thereto, a lot of improvement is necessary.
Furthermore, using an image-receiving sheet having an ink-receiving layer comprising a micro-porous layer, which was formed according to the wet-solidification method as disclosed in JP, A, 62-197183, which is one of methods as disclosed in JP, A, 6-286181, the present inventors evaluated the overlapping properties of a second color, a third color and a fourth color. As a result, this type of image-receiving sheet had the disadvantage that a flow of ink and a deficiency of dot were found in a large amount of dots from an intermediate density region to a shadow region. In order to provide a color image having a high-quality, it is very important to evaluate not only the quality of a first color but also overlapping properties of a second color and subsequent color(s). However, as far as the present inventors know, none of conventional technologies as publicly known refer thereto.
It is an object of the present invention to provide an ink image-receiving sheet used in a melting type of thermal transfer recording system, which is excellent in receiving property of transferred ink, and capable of forming thereon an image of a high quality and a high-definition which is excellent in dot reproducibility and gradation property, in particular when a full-color printing is carried out.
Another object of the present invention is to provide an ink image-receiving sheet having a surface layer wherein a lot of pores having an average pore size of preferably 1 to 5 xcexcm and a small variation in size and shape are formed.
A still another object of the present invention is to provide an ink image-receiving sheet which is excellent in writing property.
A further object of the present invention is to provide a method of forming an image by using the above-mentioned ink image-receiving sheet having an excellent receiving property to transferred ink, which is capable of forming thereon an image which is excellent in dot reproducibility and gradation property, in particular, dot reproducibility of a second color or subsequent color(s) when a color image is formed.
These and other objects of the present invention will become apparent from the description hereinafter.
The present inventors have eagerly investigated in order to achieve the above objects. Consequently, we have found that according to a method comprising the steps of: providing an ink image-receiving sheet comprising an ink-receiving layer having a porous structure formed according to a wet-solidification method, the ink-receiving layer containing a binder resin and aragonite-type calcium carbonate as main components; and melting a heat-meltable ink or each of a plurality of different color heat-meltable inks by heat provided through a printer head on heat-transferring to infiltrate each ink into the porous structure of the ink-receiving layer, an image which is excellent in gradation reproducibility and dot reproducibility including variable performance, in particular, dot reproducibility of a second color and subsequent color(s) when a color picture image is formed, and color clearness can be realized; and that the ink image-receiving sheet is excellent in writing property.
That is to say, the present invention set forth in claim 1 provides an ink image-receiving sheet for use in a method of forming an image, comprising an ink-receiving layer having a porous structure formed according to a wet-solidification method, the ink-receiving layer comprising a binder resin and aragonite-type calcium carbonate as main components.
The present invention set forth in claim 2 provides the ink image-receiving sheet of claim 1, wherein a weight ratio of the binder resin to the aragonite-type calcium carbonate is in the range of 70:30 to 35:65, and a rate of porosity of the ink-receiving layer is in the range of 30 to 70%.
The present invention set forth in claim 3 provides the ink image-receiving sheet of any one of claims 1 or 2, wherein the binder resin comprises at least one member selected from the group consisting of a polyester resin, a poly (vinyl butyral) resin, a poly (vinyl acetal) resin, a polyurethane resin, a styrene resin, an acrylonitrile copolymer, a vinyl chloride resin, a vinyl acetate resin and a styrene-butadiene copolymer.
The present invention set forth in claim 4 provides the ink image-receiving sheet of any one of claims 1 to 3, wherein the binder resin contains at least a polyurethane resin.
The present invention set forth in claim 5 provides a method of forming an image, comprising the steps of: superimposing an ink image-receiving sheet having a porous ink-receiving layer and a heat-meltable ink on a thermal transfer sheet; and melting the ink by heat provided through a thermal transfer printer head from the back side of the thermal transfer sheet, thereby infiltrating the ink into the porous ink-receiving layer of the ink image-receiving sheet to form an image, wherein the ink image-receiving sheet comprises an ink-receiving layer having a porous structure formed according to a wet-solidification method, the ink-receiving layer comprising a binder resin and aragonite-type calcium carbonate as main components.
The present invention set forth in claim 6 provides a method of forming an image, comprising the steps of: superimposing an ink image-receiving sheet having a porous ink-receiving layer and each of a plurality of different color heat-meltable inks on a thermal transfer sheet or on a plurality of thermal transfer sheets; and melting each ink by heat provided through a thermal transfer printer head from the back side of the thermal transfer sheet, thereby successively infiltrating the plurality of different color inks into the porous ink-receiving layer of the ink image-receiving sheet to form a color image, wherein the ink image-receiving sheet comprises an ink-receiving layer having a porous structure formed according to a wet-solidification method, the ink-receiving layer comprising a binder resin and aragonite-type calcium carbonate as main components.
According to the present invention, it has been found that a coating liquid comprising a binder resin and an aragonite-type calcium carbonate is applied onto one side of a substrate, wherein the aragonite-type calcium carbonate is dispersed in the binder resin, the binder resin is preferably one or more selected from the group consisting of a poly (vinyl acetal) resin, a poly (vinyl butyral) resin, a polyurethane resin, a polyester resin, and the like, which resins are commonly used in the wet-solidification method, and the aragonite-type calcium carbonate is pulverized and dispersed in an average particle size of preferably 2.5 xcexcm or less, more preferably 1.5 xcexcm or less; and the applied coating is made into a porous structure by the wet-solidification method, whereby pores uniform in shape and dense can be formed in an ink-receiving layer. Furthermore, it has been found that when a full-color printing is carried out by using different color heat-meltable inks (e.g. yellow ink, magenta ink, cyan ink, and the like) each containing a wax as a main component on an image-receiving sheet as produced according to this method, even a second color ink and subsequent color ink(s) can be sufficiently infiltrated into the porous ink-receiving layer; a color image having a rich gradation representation and a high definition which could not be represented by the conventional area gradation method can be formed, wherein a jumping-up of a density near a saturated density in a high density region is not caused; fine dots having a uniform shape in a low density region can be reproduced. Besides, it has been found that by using an aragonite-type calcium carbonate, the hardness of the surface of the ink-receiving layer is increased, and writing properties, in particular writing properties in pencil are improved.
Hereinafter, the present invention will be described in detail.
In the present invention, as a substrate for an ink image-receiving sheet, a common plastic film or sheet, a white plastic film or sheet prepared by addition of a white pigment or the like to a plastic, a foamed plastic film or sheet, a synthetic paper sheet, a paper sheet such as laminated paper sheet which is provided with water resisting property, and the like can be used. Furthermore, a primer layer may be provided to these materials in order to improve adhesive property between an ink-receiving layer and the substrate, or an electrostatic protection treatment or a corona treatment may be applied thereto. Besides, if necessary, a layer for providing sliding property or an antistatic layer may be provided on the rear side.
A coating liquid for an ink-receiving layer is applied onto the surface of the substrate for an image-receiving sheet and the resultant coating layer is made into an ink-receiving layer having a porous structure according to a wet-solidification method.
First, the wet-solidification method will be explained. In the wet-solidification method, a coating liquid for an ink-receiving layer in which a binder resin is dissolved in an organic solvent is applied onto a substrate, and thereafter the substrate with a coating layer of the coating liquid is immersed into a bath containing a treating liquid which is compatible with the organic solvent above, and in which the binder resin above is not dissolved. The organic solvent in the coating layer and the treating liquid in the bath are highly compatible with each other, and therefore, a substitution between both is caused, the organic solvent in the coating layer is outflowed into the bath, while the treating liquid in the bath is infiltrated into the coating layer, wherein the binder resin in the coating layer is not dissolved in the treating liquid infiltrated from the bath, and thus solidified. Thereby, sites from which the solvent in the coating layer has been removed form voids, and thus a porous layer is produced. In this respect, JP, B, 49-25430 and JP, B, 5-18332 discloses a method wherein fine particles as a third composition are added to a coating liquid for an ink-receiving layer; and JP, B, 5-87311 discloses a method wherein two or more resins which are low in miscibility with each other can be used as a binder resin.
A coating liquid for an ink-receiving layer as used in the present invention comprises one or more binder resins as a main component, an organic solvent, and an aragonite-type calcium carbonate, wherein the binder resin(s) is dissolved in the organic solvent, and the aragonite-type calcium carbonate is dispersed therein. Hereinafter, each of the components will be explained.
In the present invention, as a binder resin used in a coating liquid for an ink-receiving layer, the following are used: polyester resins, poly(vinyl butyral) resin, poly(vinyl acetal) resin, polyurethane resins, styrene resins, acrylonitrile copolymers, vinyl chloride resins, vinyl acetate resins and styrene-butadiene copolymers, and the like. These binder resins may be used alone, or may be used in combination of two or more species thereof. In particular, from the view point of easy formation of pores, the binder resin preferably contains at least a polyurethane resin. Examples of the polyurethane resins usable in the present invention include polyester type polyurethane resins, polyether type polyurethane resins, polyamide type polyurethane resins, polycarbonate type polyurethane resins, and the like. However, from the view point of carrying out the wet-solidification method, polyester type polyurethane resins are most desirable.
Then, an aragonite-type calcium carbonate used in a coating liquid for an ink-receiving layer in the present invention will be explained. In the case of light calcium carbonate (precipitated calcium carbonate) such as an aragonite-type calcium carbonate, which is chemically synthesized, a lot of primary particles are usually bonded with each other to form a secondary agglomerate. However, in the present invention, an aragonite-type calcium carbonate may be dispersed in a binder in such a state that an agglomerate structure remains, or may be dispersed in the binder in such a state that an agglomerate structure is broken into an optional size. However, it is preferable to use an aragonite-type calcium carbonate in such a state that it is pulverized and homogeneously dispersed in the binder by a dispersing machine using media till the agglomerate structure is broken (till the aragonite-type calcium carbonate has an average particle size of 2.5 xcexcm or less, preferably 1.5 xcexcm or less). In the case that the agglomerate structure is not sufficiently broken, a secondary agglomerate tends to emerge on the surface of an ink-receiving layer which is formed according to the wet-solidification method, and thus smoothness of the surface cannot be sufficiently provided. Consequently, it becomes difficult to form dots having a uniform shape, and a rough appearance is caused in a highlight region due to a deficiency or a loss of fine dots. Therefore, it becomes difficult to form a high-definition image.
As a dispersing machine using media, vessel type mills such as attritor and centri mill; flow cylinder type mills as sand grinder, dyno mill, grain mill, pearl mill or matter mill; annular type mills such as conical ball mill; and the like can be preferably used. Examples of media used on dispersing include natural or synthetic fine particles such as flint stones, Ottawa sands, steel balls, alumina balls, zirconia beads or glass beads can be enumerated. A vessel is filled with these media, the media are mechanically agitated through agitating blades having a predetermined shape, while a liquid to be dispersed is passed one or more times, thereby pulverizing an aragonite calcium carbonate and dispersing the resultant into the binder.
The weight ratio of a binder resin to an aragonite calcium carbonate is preferably in the range of 70:30 to 35:65. When the ratio of the binder resin is larger than this range, the effect of the aragonite calcium carbonate on forming pores, in particular the effect thereof on forming pores in the surface of an ink-receiving layer is decreased. Thus, when a printing using a heat-meltable ink is carried out, an infiltration of ink into the ink-receiving layer is decreased, and the shape of dots is deteriorated. When the ratio of the aragonite calcium carbonate is larger than this range, the strength of the surface of the ink-receiving layer is decreased, and particles fall out even due to a weak frictional force. Consequently, writing properties are decreased.
Furthermore, in order to control the shape of pores, various surface active agents or other additive agents may be added to a coating liquid for an ink-receiving layer.
Then, the organic solvent which is used in a coating liquid for an ink-receiving layer and the treating liquid in the present invention will be explained. With respect to the solvent, various organic solvents which have a high power for dissolving a resin used as a binder in the present invention and are highly compatible with a treating liquid used in a bath in the present invention can be used. Examples of solvents used in the present invention include N, N-dimethylformamide, N, N-dimethylacetamide, dimethyl sulfoxide, tetrahydrofuran, tetramethylurea, N-methyl-2-pyrrolidone, ethyl acetate, dioxane, butylcarbinol, toluene, phenol, chloroform, xcex3-butyrolactone, and ketones and alcohols. Among them, N, N-dimethylformamide is commonly highly compatible with a suitable treating liquid such as water, and at the same time has a high power for dissolving a lot of resins, and thus is a suitable organic solvent in the wet-solidification method. As the treating liquid used in the bath, taking into consideration compatibility with these solvents and incompatibility with the binder resins, the following are suitably selected for use: water, acetone, ethanol, toluene, tetrachloroethylene, methyl isobutyl ketone, chloroform, hexane, and the like.
A coating liquid for an ink-receiving layer is applied onto a substrate. As a method of applying the coating liquid, the following various conventional methods can be used: reverse roll coating, air knife coating, gravure coating, blade coating, comma coating, rod coating, and the like. A substrate with a coating layer of the coating liquid for the ink-receiving layer is immersed into a bath so as to make the coating layer into a porous layer, giving an ink-receiving layer.
Thus, an ink image-receiving sheet in which an ink-receiving layer includes a lot of pores having an average pore size of preferably 1 to 5 xcexcm and a small variation in size and shape can be provided. Regarding the ink-receiving layer, a ratio occupied by pores in the layer, namely a rate of porosity is preferably in the range of 30 to 70%. The rate of porosity can be determined by the following formula:
Rate of porosity (%)=(1xe2x88x92w/dh)xc3x97100 
wherein
w: Dry weight of ink-receiving layer (g/m2),
d: True specific gravity of ink-receiving layer (g/cm3), and
h: Thickness of ink-receiving layer (xcexcm).
When the rate of porosity is larger than this range, the strength of the ink-receiving layer itself is decreased, and thus when a printing is carried out, the layer cannot be restored into original shape by itself due to pressure pushed by a thermal printer head, and thus when in particular a second color and subsequent color(s) are printed, the thermal printer head cannot come into close contact with the ink-receiving layer, and thereby a heat-meltable ink is not sufficiently melted, and it becomes difficult to form uniform dots, in particular in a low density region. Furthermore, when the rate of porosity is smaller than this range, the flexibility of the layer is decreased, namely, the cushion property is decreased, and thereby as shown when the rate of porosity is larger, the thermal printer head cannot come into close contact with the ink-receiving layer, resulting in a decreased dot reproducibility. Furthermore, since the volume of pores in the layer is decreased, a sufficient amount of heat-meltable ink cannot be infiltrated, and thus the shape of dots of a second color and subsequent color(s) is deteriorated. A thickness of about 5 xcexcm to about 50 xcexcm is suitable for the ink-receiving layer. If necessary, a layer for providing sliding property or an antistatic layer may be provided on the ink-receiving layer.
As the thermal transfer sheet which is used in combination with the ink image-receiving sheet, a thermal transfer sheet having a heat-meltable ink containing a wax as a main component is most suitable, wherein the ink is melted with heat provided from a printer head on heat-transferring, and infiltrated into a porous structure of the ink-receiving layer. Besides, a bridge type of heat-transfer sheet having a semi-resin type of ink which contains a resin and a wax as main components can also be used. The thermal transfer sheet may be one in which colored ink layers of a plurality of colors are repeatedly arranged on the same substrate in a side-by-side relationship, and may be one in which each of colored ink layers is arranged on a discrete substrate. Usually, the plural colors include yellow, magenta and cyan, and if necessary, black.